1. Field of the Invention
This invention relates to a process for producing an .alpha.,.beta.-unsaturated carboxylic acid polymer, and more particularly it relates to a process for producing a polymer or a copolymer of .alpha.,.beta.-unsaturated carboxylic acid(s) or salt(s) thereof. According to the present invention, polymers such as polyacrylic acid, polymethacrylic acid, polymaleic acid and alkali metal salts thereof and copolymers thereof, which are useful as dispersants or antiscaling agents, can be stably and continuously produced to give each a less-colored product showing a sharp, i.e., narrow molecular weight distribution.
2. Description of the Prior Art
As a process for continuously producing an .alpha.,.beta.-unsaturated carboxylic acid polymer. Japanese Patent Publication No. 24806/1985 has disclosed a process for the production of acrylate polymer. Further Japanese Patent Laid-Open No. 28409/1985 has disclosed a process having similar objects as those of the former.
The process described in Japanese Patent Publication No. 24806/1985 is a polymerization process which comprises continuously adding an acrylate monomer and sodium hydrogensulfite dropwise to a stirred reaction vessel while introducing finely bubbled air thereto. It is disadvantageous since a decrease in the viscosity during the latter period of the polymerization makes it significantly troublesome to control the foaming, temperature, and bubbled air size.
On the other hand, when polyacrylic acid is produced according to the process of Japanese Patent Laid-Open No. 28409/1985 in a system where persulfate(s) and hydrogen peroxide are simultaneously employed, the circulating aqueous reaction mixture may be contaminated with a large amount of bubbles caused by the evolution of a gas, which seems to mainly comprise oxygen, accompanying the decomposition of the hydrogen peroxide and carbon dioxide gas, which is a side product. Thus, the recycle of the aqueous reaction mixture is made impossible shortly after the initiation of the operation. In addition, polyacrylic acid or the like to be used as a dispersant or an antiscaling agent preferably has a sharp molecular weight distribution and a relatively low molecular weight from the viewpoint of its adsorptivity, electric charge-imparting properties, etc. However the polyacrylic acid produced according to the process of Japanese Patent Laid-Open No. 28409/1985 has a wide molecular weight distribution and a large molecular weight.
Accordingly it is the major object of the present invention to provide a process for stably and continuously producing an .alpha.,.beta.-unsaturated carboxylic acid polymer such as an acrylate polymer, which is useful as a dispersant or an antiscaling agent, to give a less colored product having a sharp, i.e., narrow molecular weight distribution.